1. Field of the Invention
The present invention relates to mobile or cellular communications and more particularly to a method and means in such communication systems for providing a shorter handoff time at a Mobile Station and an improved gain control (AGC) value in its receiver.
2. Prior Art
The handoff time, i.e., the time taken when a Mobile Station is switching communication from a communication channel currently being received to a new channel in the same or another Base Station, is a critical feature in wireless or cellular phone operation. The handoff clicks or audio mutes will be heard by the user and therefore cellular operators are sensitive about this feature since an extended number of clicks can become annoying and give the impression of poor audio quality.
In accomplishing the handoff, the signal strength of the new channel must be determined in order to suitably adjust the receiver gain (AGC). Some requirements in this regard are set by the radio channel Rayleigh fading which effects the received signal strength (RSS) measurements, and, since the receiver gain (AGC) is adjusted by receiver signal strength information (RSSI), the receiver gain calculations as well. Shortening of the handoff procedure time is partly limited by the receiver Rx gain (AGC) tuning for the new radio channel, and tuning time and accuracy are impacted by the channel fading, so that the fading also effects the handoff. In particular, to deal with the fading, a longer measurement of the new or next communication or traffic channel signal is required for a more accurate AGC determination, since fading effects are minimized or eliminated by longer RSS measurements. The resulting long handoff times will lead to longer handoff clicks or mutes that are undesirably heard by the user.
The same long handoff delay phenomenon, as well as increased bit error rate (BER), may result from the use of the wrong AGC. Wrong gain (AGC) in the receiver may lead to improper signal detection and therefore increase BER or even lead to failure in synchronization in a new communication channel. The need for a new synchronization after such failure will cause extra delay.
Problem to be solved:
Presently, handoff times are typically 15-120 ms depending on the system. Reliable RSS measurement, eliminating the Rayleigh fading effects, contributes to the Rx gain adjustment quality (depending on the mobile speed) and tuning, but may require an unacceptable amount of time. Typical AGC tuning is based entirely on one, approximately 10 ms. measurement period during handoff. This time may be heard by the cellular user and additionally it may not be sufficient for accurate tuning due to the fading effects, i.e., fluctuation in the RSSI. Thus, a problem exists with regard to achieving rapid handoff and accurate initial gain values in the receiver of a Mobile Station when acquiring a new traffic or other communication channel from the old or a new Base Station.
Objects:
It is therefore desirable and an object of the present invention to provide a system with shorter handoff times in a Mobile Station and more accurate initial AGC values in its receiver.
It is a further object of the invention to use RSS measurement methods which will achieve shorter handoff times in the Mobile Station and more accurate initial AGC values in the receiver.